Conventional IQ modulator systems used to generate phase/frequency modulated data typically include a Cartesian up converter that employs a local oscillator in the form of a PLL. These systems typically require at least an RF quadrature generator, two RF mixers and two digital-to-analog converters. These systems are fairly complicated, expensive, require a lot of power, and utilize significant die area.
A more efficient way to generate modulation signals utilizes the fine phase/frequency control available with a fractional-N PLL. However, when a conventional fractional-N PLL is utilized at higher data rates, the noise, e.g., quantization noise, introduced by the increased bandwidth of the fractional-N PLL degrades modulation quality. The loop filter of the fractional-N PLL also has the conflicting requirement that it needs to act as a high bandwidth filter to accommodate high data rates and as a low bandwidth filter to remove quantization noise.
One conventional technique that attempts to increase modulation bandwidth beyond the loop bandwidth of the PLL while reducing unwanted noise includes two-port modulation. A typical conventional two port PLL RF modulator system includes a first modulation port introduced at the N-divider circuit that is responsive to modulation data and has a low pass bandwidth characteristic. A second modulation port, also responsive to modulation data, is typically introduced between the loop filter and the voltage controlled oscillator (VCO). The second modulation port has a high pass characteristic that allows the PLL to accommodate higher data rates. The system attempts to match the gain of the two modulation ports in such a way that the data is not bandwidth limited.
Typically, the gain associated with the first modulation port is relatively predictable due to its digital nature. However, the gain associated with second modulation port is often not as expected due to manufacturing processes, temperature variations, supply voltage variations, and the like, of the various components of the PLL. Hence, the gain of the second modulation port will typically not match the gain of the first modulation port, causing the PLL loop gain to vary significantly resulting in system malfunctioning. Therefore, accurate calibration of the PLL loop gain and/or the gain of the second modulation port is needed.
Conventional PLL RF modulator systems and methods that attempt to overcome the problem associated with gain matching of the modulation ports of a two-port PLL RF modulator often utilize a measure and adjust approach. The PLL is programmed to one frequency and the input control voltage of the VCO is measured to provide an estimate of the loop gain of the PLL. The PLL is then programmed to a second frequency and the input control voltage is measured again. The measured voltage difference is then used typically in a look-up table to set the correct modulation gain of the second modulation port. However, these conventional systems provide no feedback and cannot determine if the gain adjustment is compensated as required. The systems require at least a one time calibration that requires taking the system offline which is cumbersome and expensive. Once set, the system can go out of adjustment and/or change due to temperature variations and the like. Yet another conventional PLL RF modulator that utilizes the benefits of feedback samples the VCO output in quadrature. The design requires an additional loop placed around the PLL which compares the output RF phase with an estimate of the desired phase due to the data and the channel to correct the loop gain variation. The drawback of this design is the need for RF phase sampling and quadrature, which is expensive and complicated.
Once the loop gain of a PLL RF modulator system is calibrated, a technique known as pre-emphasis can be utilized to increase the modulation bandwidth. Pre-emphasis employs a digital filter responsive to the modulation data which is constructed with the inverse transfer function of the low pass PLL transfer function. The output of the digital filter is typically input to the first modulation port. One advantage of the digital filter is that it generates less noise. Other advantages of a digital filter are known by those skilled in the art. However, the digital filter must match the PLL response.